IV. (1.21) All these things, then, we feel: but the heaven has a nature which is incomprehensible, and it has never conveyed to us any distinct indication by which we can understand its nature; for what can we say? that it is solid ice, as some persons have chosen to assert? or that it is the purest fire? or that it is a fifth body, moving in a circle having no participation in any of the four elements? For what can we say? Has that most remote sphere of the fixed stars any density in an upward direction? or is it merely a superficies devoid of all depth, something like a plane figure? (1.22) And what are the stars? Are they masses of earth full of fire? For some persons have said that they are hills, and valleys, and thickets, men who are worthy of a prison and a treadmill, or of any place where there are instruments proper for the punishment of impious persons; or are they, as some one has defined them, a continuous and dense harmony, the closely packed, indissoluble mass of aether? Again, are they animated and intelligent? or are they destitute alike of mind and vitality? Have they their motions in consequence of any choice of their own? or merely because they are compulsory? (1.23) What, again, are we to say of the moon? Does she show us a light of her own, or a borrowed and illegitimate one, only reflected from the rays of the sun? or is neither of these things true, but has she something mixed, as it were, so as to be a sort of combination of her own light and of that which belongs to some other body? For all these things, and others like them, belonging to the fourth and most excellent of the bodies in the world, namely, the heaven, are uncertain and incomprehensible, and are spoken of in accordance with conjectures and guesses, and not with the solid, certain reasoning of truth, (1.24) so that a person might venture to swear that no mortal man will ever be able to comprehend any one of these matters clearly. At all events, the fourth and dry well was called the well of the oath on this account, because the search after the fourth element in the world, that is to say the heaven, is without any result, and is in every respect fruitless.